Add statement to 'beforeSave' method to allow app:config:import

Author: Mateusz Lerczak

app/code/Magento/Braintree/Model/Adminhtml/System/Config/CountryCreditCard.php

@@ -66,6 +66,13 @@ public function __construct(
     public function beforeSave()
     {
         $value = $this->getValue();
+        if (!is_array($value)) {
+            try {
+                $value = $this->serializer->unserialize($value);
+            } catch (\InvalidArgumentException $e) {
+                $value = [];
+            }
+        }
         $result = [];
         foreach ($value as $data) {
             if (empty($data['country_id']) || empty($data['cc_types'])) {